A diode array spectrophotometer with an entrance slit apparatus, a diffraction grating and a diode array images the entrance slit of the entrance slit apparatus onto the diode array via the diffraction grating. Beside the spectral division of light, the diffraction grating typically also has an imaging function corresponding to that of a concave mirror.
The photodiode array of a diode array spectrophotometer has a typical length of between about 12.5 mm and 25 mm, diode array lengths of about 25 mm normally being employed with higher-resolution instruments. A UV/VIS diode array spectrophotometer covers a wavelength range of 200 nm to 800 nm, giving a linear dispersion of about 40 nm/mm. Many fields of application require a wavelength reproducibility of less than 0.05 nm with a wavelength accuracy of better than 1 nm across a wide temperature range. This means that the mechanical and thermal stability of the arrangement of the entrance slit relative to the diffraction grating and to the diode array within the operating temperature range must lie in the submicrometer range, even when shocks and vibrations occur, as they unavoidably do in everyday use.
With a diode array spectrophotometer, it is also necessary while assembling the instrument to make adjustments to the positioning of the optical elements relative to one another, i.e. of the entrance slit relative to the diffraction grating and to the diode array. After conducting adjustments, the adjusted elements need to be fixed in such a manner that the mechanical and thermo-mechanical stability described above is achieved.
In order to minimize geometrical displacements due to the differing coefficients of thermal expansion of the optical components of a diode array spectrophotometer, U.S. Pat. No. 4,709,989 teaches that the spectrophotometer's casing can be made from a compressed ceramic material whose coefficient of thermal expansion is adapted to the coefficient of thermal expansion of the lenses used in the optical elements of the spectrophotometer.
U.S. Pat. No. 4,805,993 discloses an additional diode array spectrophotometer with a slit, a diode array and an optical grating, which are held relative to one another by a casing. For the purposes of adjusting and fixing the grating relative to the casing, the grating has a peg at the back, which is held by a manipulator during adjustment. The grating can be moved along the x and y axes using the layout plate and can also be rotated about these axes. To enable displacement in the z direction, the grating can be moved within the cylindrical opening in the layout plate. When the final adjustment position has been reached, the layout plate together with the grating is fixed relative to the casing using a self-hardening substance. The grating and the layout plates of the known diode array spectrophotometer are made of glass or ceramic. The structure described for adjusting and fixing the diffraction grating of the known diode array spectrophotometer using the plate arrangement described is complicated both to manufacture and to handle during the adjusting and fixing stages. With this known diode array spectrophotometer, a fine adjustment of the diode array must be carried out after the described adjustment of the grating, since not all degrees of freedom are available for adjusting the grating.